Separation and purification of cyclodienes



Jan. 6, 1959 L. w. MCLEAN ET AL 2,867,670

SEPARATION AND PURIFICATION oF cYcLonIENEs Filed March 2, 1954 REcYcLE or 09+ LAWRENCE VV. MLEAN |NVENT0R3 L EON C. KENYON JR.

BY MM-. ATTORNEY @tats ass/,aio

SEPARATION AND PURIFICATION F` CYCLODIENES Application March 2, 1954, Serial No. 413,615

4 Claims. (Cl. 260-666) This invention relates to separation and purification of cyclopentadiene and methylcyclopentadiene monomers which are mixed in a vapor stream from their cracked crude dimers.

The fractionation of the mixed C5 and C5 cyclodiene monomers contaminated by higher boiling hydrocarbons is complicated by the tendency of these monomers to redimerize and polymerize. The rapid redimerization makes necessary recycling of dimers formed in the distillate to the cracking zone. In the recycling there is a problem of preventing formation of polymers which resist cracking to monomers. There are also other prob? lems of preventing the formation of polymers which tend to cause plugging of the equipment.

In accordance with the present invention, difiiculties from dedimerizing and polymerizing to higher molecular weight polymers are advantageously overcome by first.

fractionating C5 and C5 monomers together from higher boiling components of a cracked dimer product under conditions to provide suitable bottoms for recycling to a cracking zone, and then by further fractionating the C5 and C5 monomers from the first fractionation zone in a second fractionation zone wherein reboiling conditions are maintained to bring about cracking at a rate substantially equivalent to the dimerization rate. By this combination of fractionation steps there is a minimized loss of cyclodienes in purged material unsuitable for recracking. By material unsuitable for recracking is meant a mixture of the C5 and C5 cyclodiene dimers with other codimers and polymers that tend to give rise to substances difficult to recrack and which tend to cause plugging in the equipment.

A preferred mode of operation will be described with reference to the schematic flow diagram in the drawing which illustrates principal equipment used in conjunction with a crude cyclodiene dimer purification unit designed to produce cyclopentadiene and methylcyclopentadiene products, each of above 95% purity.

In the drawing, the first and second fractionating towers, F-l and F-2, are connected in a series to the cracking unit C-3. The cracking unit may be a liquidphase, vapor-phase, mixed-phase, or any other type of cracking unit supplying a vapor mixture that contains a substantial amount of C5 and C5 cyclodiene monomer mixed with higher boiling hydrocarbons, principally in therange of C7 to C12.

In using an equilibrium liquidephase cracking zone in cracking unit C-3, a fresh feed stream was charged from line, 4. This stream was a C5 and C5 cyclodiene dimer concentrate which should theoretically yield about 30% to 40% cyclopentadiene and 30 to 38% methylcyclopentadiene.

The cracking zone was heated electrically and was operated to give a 1 to 2 hour holdup at about 400 F. to 420 F. at pressures from atmospheric to about 2 atmospheres.

The fresh feed stream mixed with a recycle stream from line 5 was preheated to about 320 F. in the heat eX- changer 6. Vapor products of the cracking are released Patented Jan. 6, 1959 ICC under conditions and with sufficient fractionation to take the C5 and C5 monomer vapors overhead through line 11 to the second fractionation zone in column F-Z.

The overhead vapor stream from the first fractionating column or tower F-l may be maintained at a vapor temperature of about 136 F. to 138 F. At these temperatures monomers of C5 cyclodienes, about 57%, and C5 v cyclodienes, about 40%, with less than 1.0% C7 cyclodienes are passed on to the second fractionating tower F-2. With this kind of feed to the second tower F-Z, the C5 and C5 monomers can be satisfactorily separated and recovered as purity products with the C7 cyclodienes in the C5 cyclodiene stream as an impurity.

The C5 cyclodiene product is taken as an overhead fraction from F-2 through line 12. The C5 cyclodiene product is withdrawn as a side stream through line 13 or as a bottoms fraction by line 14 from tower F-Z. Condensing heat exchangers 15 in overhead line 11, and 16 in overhead line 12 can be used to provide refiux which is returned to the top of each of the towers F51 and F-2 so as to aid the control of vapor temperatures at the top of each of these towers or columns; for example, a temperature of the order of 135-l40 F. at the top of F-1 and a temperature of the order of l00ll0 F. at the top of F-2. A significant difference in the operation of towers F-l and F-2 is in the necessity of recycle from` the F-l tower, whereas the F-2 tower can be operated without recycle or purge losses.

In the operation of the first fractionating tower F-l, it is important to make a separation between the heavier components carried over with the C5 and C5 monomers. This separation is advantageously carried out by withdrawing a Cfr-C9 purge as a side stream between the inlet from line 7 and the'bottom of tower F-l. This side stream may amount to about 10% of the fresh feed and is indicated to be withdrawn through line 17. This purge material containing mostly CT-Cg hydrocarbons is preferably prevented from returning to the cracking zone with redirnerized material which is to be recycled as bottoms from tower F-1 through line 5. The recycle bottoms stream with the type of operation described should generally amount to about 400 to 500% of the fresh feed rate. It is necessary to recycle this large proportion of material and to subject it to recracking on account of the equilibrium conditions in the cracking zone and on account of the dimerization rate in the first fractionating zone. By using a proper recycle rate excessive polymerization in the cracking zone is avoided.

Operations have demonstrated that the second column F-2 can be controlled to recrack C5 -and C5 cyclodiene dimers to give close to v% recovery of the desired monomervfed to column F-2. Accordingly, recycling from the bottom of the second column F-2 can be entirely eliminated with the advantage that desirable cyclodiene products to not have to be recycled to the cracking zone, where a portion would become part of the purge stream from the cracker. -1

Reliux and product takeoff systems, on both fractionating towers were hand controlled by rotometer sets but any equivalent means may be used for controlling temperatures. The desired monomer products from` the second tower, F-Z, were batched dimerized for storage.

Although the main features of operation have been emphasized in the description of this point, there are several other control features which are important in the process for obtaining better recovery of desired products. For example, if complete separation of C from C7 compounds is attempted in the first fractionator F-1, a significant portion of C5 tends to be lost due to the diiculty of making this separation. Thus, Vit was found desirable to allow an appreciable small portion of C7 (1/2 to 11/2 Weight percent) to remain in the overhead of C5 and C5 monomers from tower F-l passed to tower F-2.

This small amount of C7 taken overhead will tend to give some contamination to the C5 product withdrawn from tower F-2. However, with proper operating conditions in the towers, the purity of methylcyclopentadiene side stream withdrawn from the second tower F-2 remains above 95%.

`It was proved that the problem of excessive polymerization to more refractory materials in the fractionators F-l and F-2 was well overcome. A system, such as described, was operated for a period of weeks without plugging difficulties. Inspection of the unit showed that the reboilers with heating means 21 and 22 of both towers were in excellent condition with no coke or gummy deposits. The foregoing more particular studies demonstrate the teaching that it is desirable to permit a small amount of the C7 components to pass overhead with the C5 and C5 monomers from the first fractionating zone F-1. These studies also demonstrated that it was desirable to have substantially all the recycle from the bottom of the first tower F-1 for recracking and to preferably maintain a recycle rate of about 400 to 450% with release to the fresh feed supplied to a liquid phase cracking unit. One of the most important factors as previously pointed out in the described operation,`is in maintaining a sufiiciently high reboiler heat input through heating means 22 to the bottoms of the second tower F-Z so as to recrack the dimer at a rate equivalent to the dimerization in the tower. It was demonstrated that the desired operation was accomplished by reboiler temperatures of the order of 350 to 400 F. under atmospheric conditions and that these reboiler conditions did not adversely interfere with a satisfactory separation of the C5 cyclodiene monomer product having a purity above 95% and containing no more than about 2.5% of C7 impurity. A typical operation is summarized in the following tabulated form.

.9-1.2 hours hold up operating conditions:

136138 F. overhead vapor temperature S20-360 F. reboiler temperature 450% recycle to cracker 5/1 reliux ratio yAtmospheric pressure C7-C9 sidestream purge, 10% based on fresh feed F-Z operating conditions:

,105 F. overhead vapor 4temperature 160 F. C5 side Astream S50-400 F. reboiler temperature No recycle S/l reflux ratio Atmospheric pressure No purge monomers contaminated by C7 to C5 hydrocarbon im` purities in a vapor stream, first fractionating said stream to separate the C5 and C5 monomers with about 1/2 to 11/2 weight percent of C7 impurities as an overhead distillate from a first fractionating zone having a reboiling bottoms temperature of 320 to 360 F., then separating the C5 and C5 monomers of said overhead distillate in a second fractionating zone having a bottoms temperature in the range of 350 to 400 F. to crack substantially all dimer formed in said second fractionating zone which is maintained substantially free of C7 to C9 hydrocarbon impurities except for the small amount of C7 'impurities present inthe overhead distil` late from said first fractionating zone.'

2. In a process of claim 1, withdrawing a side stream of C7 .to C9 impurities from the first fractionating zone.

3. In fractionating a mixture of C5 and C5 'cyclodiene monomers contaminated by C7 and higher boiling hydrocarbons, the improvement which comprises first fractionating said mixture kto `obtain la mixture of the C5 and C5 cyclodiene 'monomers as a distillate freed of higher boiling hydrocarbons except for C7 hydrocarbons in an amount up to 1-11/2 weight percent, passing the resulting distillate fraction of the C5 and C5 cyclodiene monomers thus freed of the higher boiling hydrocarbons to a second fractionating zone, maintaining a high reboiling heat input in the bottoms part of said fractionating zone to crack dimers of the cyclodienes at a purities in a vapor stream from a cracking zone, lirst fractionating said stream to separate the C5 and C5 monomers with about 0.5 to 1.5 weight percent C7 impurities in an overhead distillate vapor at a temperature of about 136 F. to 138 F. from a first fractionating zone having a reboiling bottoms temperature of 320 F. to 360 F., a portion of the overhead distillate being reiiuxed to the upper part of said rst fractionating zone for temperature control, withdrawing a side stream purge of C7 to C9 impurities from said first fractionating zone, recycling bottoms of the rst fractionating zone to the cracking zone in a proportion of about 400 to 500% of fresh feed thereto to yform said vapor stream, and sep` arating the C5 and C5 monomers of the overhead distil-A late from the first fractionating zone in a second fractionating zone having a bottoms temperature in the range of 350 F. to 400 F. to crack substantially all dimer formed in said second fractionating zone, said second fractionating zone being maintained substantially free of C7 to C5 hydrocarbon impurities except Vfor the small amount of C7 impurities present in the overhead distillate from Vsaid rst fractionating zone.

References Cited in the file of this patent I' UNITED STATES PATENTS Nelson et al. June l19, 195,6 

4. IN SEPARATING AND PURIFYING C5 AND C6 CYCLODIENE MONOMERS CONTAMINATED BY C7 TO C9 HYDROCARBON IMPURITIES IN A VAPOR STREAM FROM A CRACKING ZONE, FIRST FRACTIONATING SAID STREAM TO SEPARATE THE C5 AND C6 MONOMERS WITH ABOUT 0.5 TO 1.5 WEIGHT PERCENT C7 IMPURITIES IN AN OVERHEAD DISTILLATE VAPOR AT A TEMPERATURE OF ABOUT 136* F. TO 138* F. FROM A FIRST FRACTIONATING ZONE HAVING A REBOILING BOTTOMS TEMPERATURE OF 320* F. TO 360* F., A PORTION OF THE OVERHEAD DISTILLATE BEING REFLUXED TO THE UPPER PART OF SAID FIRST FRACTIONATING ZONE FOR TEMPERATURE CONTROL, WITHDRAWING A SIDE STREAM PURGE OF C7 TO C9 IMPURITIES FROM SAID FIRST FRACTIONATING ZONE, RECYCLING BOTTOMS OF THE FIRST FRACTIONATING ZONE TO THE CRACKING ZONE IN A PROPORTION OF ABOUT 400 TO 500% OF 